Poor light output or transmittance efficiency is a big problem in the light-managing industry. As you would expect, prior formulators have not contemplated using hazy or cloudy materials as optical encapsulants, waveguides or connectors. Logically, they match the refractive indexes of the constituents of their formulations to maximize their transparency.
Refractive index-matched materials for LED packages are mentioned in US 2012/0235190 A1 to B. Keller. Keller is concerned with a problem of light scattering in a light emitting diode (LED) package containing an encapsulant containing a thixotropic agent having an index of refraction that is different from that of the encapsulant. He said that untreated fumed silica can have an index of refraction of approximately 1.46, and can be mixed in a conventional encapsulant such as a silicone that has an index or refraction of 1.51 or more. Keller states that this difference in index of refraction between the thixotropic agent and the encapsulant can result in the encapsulant exhibiting scattering characteristics for the light passing through the encapsulant from the LED. The refractive index mismatch gave the encapsulant a cloudy (i.e., not clear) appearance, which can reduce emission package efficiency by reducing total light output from the package. Keller's solution is an LED package containing an encapsulant with refractive index matched thickening or thixotropic agent. The LED package exhibits a reduction or elimination of encapsulant clouding and an increase in package emission efficiency. This allows the thickening or thixotropic agent to alter certain properties (e.g. mechanical or thermal) while not significantly altering the optical properties of the encapsulant.
Refractive index matching of constituents of optical formulations is the standard not just for optical encapsulants, but also for waveguides and connectors. Formulators use refractive index matching to maximize light transmittance in waveguides as described in U.S. Pat. No. 7,551,830 B2 to J. DeGroot et al. and in optical connectors as mentioned in U.S. Pat. No. 5,783,115 to Z. Bilkadi et al.
Refractive index matching is also used for solar modules in WO 2010/141697A2 to B. Ketola et al. Ketola et al. are concerned with varying optical properties of the components of a photovoltaic module based on selection or manipulation of the material. In one embodiment, it may be desirable to select a superstrate material such as glass and an encapsulant material that provide a small (i.e., <0.05) mismatch between their refractive indices to reduce the amount of reflection at the glass-encapsulant interface. The encapsulant may comprise a silicone material having a refractive index within a specific range of that of glass such as within ±0.05 of glass refractive index. In another embodiment, the refractive index of the encapsulant material is selected to be greater than the refractive index of the superstrate material to reduce escape cone losses as described therein.
Refractive index matching is used for display devices in KR 2014-085250 A to K. S. Beom et al. Beom et al. are concerned with a problem of an organic plastic substrate for certain display devices exhibiting yellowness and cloudiness or haze. They use a composite sheet of a polysiloxane-polycarbonate copolymer matrix and a reinforcing material, wherein the difference in refractive index of the reinforcing material and that of the matrix can be less than 0.01. The composite sheet had improved transparency, optical permeability, flexure resistance, and surface strength.
As you can see, prior formulators viewed hazy or cloudy optical materials as problems to be avoided. To maximize transparency at room temperature and above, they discarded hazy or cloudy materials and matched refractive indices of the constituents of the materials. Their cloudy or hazy material, which used an untreated filler, might slightly obscure, but could not hide, phosphors in an off-state optical device. And the untreated filler would decrease cracking resistance of the material after thermal aging, and thus it might decrease light transmittance of the material and light output or transmittance efficiency of the optical device in its on-state.
Other formulators used camouflage to trick the eye into overlooking an unattractive feature. A technique for camouflaging phosphors in off-state LED devices is mentioned in US 2014/0203316 A1 to W. J. Ray. Ray is concerned with a problem of adjusting the off-state color of LED light emitting structures containing a phosphor layer, such as an yttrium aluminum garnet (YAG) phosphor layer, so the off-state color is more aesthetically pleasing. In the off-state, white ambient light, such as sunlight, impinges on the phosphor layer and energizes the phosphor particles so the entire surface of the light emitting structure appears yellow. Ray uses an LED die that contains a blue LED and a mixture of a transparent binder, yellow phosphor powder, magenta-colored glass beads, and cyan-colored glass beads. Ray prints the mixture over the light emitting surface and cures it to form a wavelength conversion layer. When the LED dies are powered on, the combination of the yellow phosphor light and the blue LED light creates white light. When the LED dies are powered off, ambient light, such as sunlight, causes the conversion layer to appear to be a mixture of yellow light, magenta light, and cyan light. Ray selects the percentage of magenta and cyan beads in the mixture to create a desired off-state color, such as a neutral color, for the conversion layer for aesthetic purposes. The phosphors in Ray's device are visible, but camouflaged and thus harder for observers to distinguish separately as such.